Parallax is a simple illusion that you need to understand in order to read your gauges correctly.
Flight instruments generally use pointers that indicate something based on their position over a card with the values printed on it.
For design and construction reasons there is often a gap of at least a few millimeters between the pointer and the values. This gap doesn’t matter when you look at the instrument straight-on. See the image of an HSI below showing a heading of 269 degrees.
It looks fine to me and it is easy to read from the left seat.
Now, look at the same instrument from the right seat, pictured below. Notice that the needle is pointing at about 263 degrees.
This is easy to mentally correct for if you expect it to display this way. You can estimate the real value it is indicating and look for the line that parallels the pointer (The line indicating west in this picture). You can also just lean over to get a better look.
Finally, know that from the right seat you will see most of the planes indications this way, so don’t expect to be as precise as you would be in the left seat.
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